Optics that focus on manufacturing

Incorporation of a special fisheye lens lets a standard LCD projector beam realistic images over more than half a sphere. Engineers from Edmund Optics worked with Elumens to redesign lens and housing elements to make the fisheye work better and be easier to manufacture. Among other things, attention to tolerances and materials paid dividends here.

Kenneth J. Barber Jr.Edmund Industrial OpticsBarrington, N.J.

It has become a truism that designs today must be readily manufacturable. But the road to manufacturability may not be obvious when it comes to multielement lenses. There are often subtleties involved in such devices. One consequence is that it may not be easy to see whether a working prototype will present problems when it hits production.

A working design, by itself, is not enough. An optical design is just a plan created to meet certain requirements: how large the dimensions can be and the wavelengths at which the device must work. But a design doesn't tell you how much the product will cost to make.

The key to design for manufacturing and design for cost comes out of an understanding of how glass optics are made, the mechanical structure of the device, and how the pieces are aligned, assembled, and tested. It's best to consider such key issues from the beginning because design optimization is a series of trade-offs. A prototype design may not be the easiest to manufacture or the least costly solution possible.

Many designers aren't well versed in the production process and don't understand the criteria that affect price. And designers can get locked into the technical details of the design and forget to make sure the $500/lb glass is five times as good a choice as the $100/lb glass. When making hundreds or thousands of units of a complex optomechanical assembly, a change in materials and surfaces can be costly but it also can save a lot of time and money.

Or consider the choice of metal. Beryllium (a material frequently used in the aerospace industry), for example, is super light and has amazing structural properties. This makes it an attractive material for mechanical designers. However, most commercial products don't need these properties. The cost of the raw beryllium and the cost of machining it would wreck most budgets -- if, in fact, the manufacturing shop can handle it at all. Aluminum and steel cost much less and are easier to manufacture than beryllium yet still provide the level of quality needed for most optomechanical applications.

Similarly, a simplistic view of design suggests that the fewer elements, the better. More optical elements do raise cost. But if additional elements can loosen up tolerances, the approach may reduce costs, not raise them. Would you rather make nine easy-to-manufacture parts or six difficult ones?

Tolerancing tiny dimensions

Those of us who work on optical components spend 50% or more of our design time on manufacturability and cost control. We do this largely by setting appropriate tolerances and reducing the design's sensitivity to them.

For example, a design that requires a tight tolerance on the centering of the optics will be more labor intensive and costly to make than one that allows more generous tolerances. But tolerances can be much tighter in optics than in many other mechanical systems. A system that can tolerate an optic shifting the width of a hair (75 to 100 mm) is considered to have a loose tolerance. A tight tolerance in optics would be about 20 to 35mm.

If a manufacturing shop can only cut a seat to a certain accuracy, then the tolerances must be looser than that accuracy. Thus part of the work involved in design for manufacturing goes into changing the design so it accommodates the tolerances of the manufacturing process itself.Manufacturing and assembly are easier, for example, if the design staggers the size of optics by a few millimeters, with the biggest optic on the outside. Each lens is retained within the housing. This approach provides better precision than could be had using spacers.

Eyeing a fisheye

A recent project serves as a good example of design-for-manufacturing principles at work. Users of 3D CAD and virtual-reality systems want to be immersed in a realistic environment, whether they use the system for training or for fun. So when the manufacturer of an immersive 3D projection system, Elumens Corp., Durham, N.C., designed its VisionStation product, one requirement was to project an image over more than 180° on the inside of a dome.

The project was interesting because the technical challenges of creating such a device are considerable. The system needed a special very-wide-angle fisheye lens. Readers who have looked through a fisheye lens installed in a door may recall how objects near the center of the field (say, a visitor's face) appear fairly normal, but objects near the edge of the field (say, a receding hallway) appear weirdly distorted. Such distortion in the projector would make the image look unrealistic and was unacceptable for the 3D system. So the fisheye lens had to offer relatively low distortion over the entire field of view. Also, the lens had to mount on a standard LCD projector.

The Elumens team created a good prototype design for the projector lens. But they were unsure if it was economical for the number of units the company expected to sell. Working with Elumens engineers we redesigned both the optical and the mechanical system with an eye toward mass production. Our main strength was our familiarity with manufacturing issues.

The final design for the lens used inexpensive glass without sacrificing performance, which saved money in the end. The redesign of the lens also includes changes to the mechanical interface between the lens and projector that provide better focusing than the original design.

The 3D VisionStations are in production and over 200 projectors are now in the field. The final design hit cost targets partly because it had optical and mechanical manufacturing capabilities in mind.